A power-saving and light-weight liquid crystal display device is often used as a display device in a compact information terminal such as a mobile telephone or a PDA. Examples of liquid crystal display devices include those that make use of external light as a lighting means for visualizing a latent image formed on a liquid crystal display panel, and that have an auxiliary lighting system installed on a rear or front surface of the liquid crystal display panel. An auxiliary lighting system provided on a rear surface of the liquid crystal display panel is sometimes referred to as a backlight system, while that provided on a rear surface is generally referred to as a front-lighting system.
Examples of a backlight system for a compact information terminal such as a mobile telephone include one having a light guide plate having a cathode luminescent lamp provided on a side face (a side edge, or a light-receiving surface) as used in a laptop personal computer with a relatively large-sized display screen. However, a backlight system that uses, in place of a cathode luminescent lamp, one or a plurality of light point sources which are solid light-emitting elements represented by light-emitting diodes is suitable for a compact information terminal. Herein, the backlight system is described by taking a light-emitting diode as an example of the solid light-emitting element.
There is a backlight system of a type in which a plurality of light-emitting diodes (each referred to as LED hereinafter) are provided as light sources, light emitted from the LED is introduced onto a side face (light-receiving surface) of the light guide plate and goes out from an illuminating surface which is a top surface of the light guide plate and functions as a surface light source. This backlight system has a large difference in brightness between the forward LEDS of the LEDs within the light guide plate. There have been proposed various ideas to reduce a difference in brightness between the LEDs when a plurality of LEDs are used, but there is still no complete countermeasures to overcome the problem.
In relation to the backlight system as described above, an idea of eliminating unevenness in brightness by improving a form of a groove provided on the light guide plate has been proposed by, for instance, Patent documents 1: Japanese Patent Laid-open No. 2004-200072, Patent document 2: Japanese Patent Laid-open No. 2004-6187, Patent document 3: Japanese Patent Laid-open No. 2004-227956 and Patent document 4: Japanese Patent Laid-open No. 7-72815.